Process for the production of a steel exhibiting consistently low weight loss test values

ABSTRACT

Steel strip to be used in the manufacture of carbonated beverage packs is treated to incorporate sulfur into its surface layers by diffusion during the annealing cycle that is normally conducted after cold rolling and cleaning operations. The steel is first treated with an aqueous sulfide solution, dried and then heated in a conventional annealing furnace.

United States Patent Whitton et al.

[451 Apr. 4, 1972 [54] PROCESS FOR THE PRODUCTION OF A STEEL EXHIBITING CONSISTENTLY LOW WEIGHT LOSS TEST VALUES [72] Inventors: Richard Wilfred Whitton, Figtree, New South Wales; Ronald Herbert Southall; Michael William McKenzie, both of Keiraville, New South Wales; Ralph Edward Shackleford, West Wollongong, New South Wales, all of Australia [73] Assignee: Australian Iron & Steel Pty. Limited, Sydney, New South Wales, Australia [22] Filed: July 14, 1969 [21] Appl. No.: 841,551

[30] Foreign Application Priority Data July 25, 1968 Australia ..4l21l/68 [52] US. Cl ..l48/6.24, 148/14 2,844,497 7/1958 Henricks l 48/6.24

Primary ExaminerRalph S. Kendall Attorney-Wenderoth, Lind and Ponack [57] ABSTRACT Steel strip to be used in the manufacture of carbonated beverage packs is treated to incorporate sulfur into its surface layers by diffusion during the annealing cycle that is normally conducted after cold rolling and cleaning operations. The steel is first treated with an aqueous sulfide solution, dried and then heated in a conventional annealing furnace.

4 Claims, No Drawings PROCESS FOR THE PRODUCTION OF A STEEL EXHIBITING CONSISTENTLY LOW WEIGHT LOSS TEST VALUES This invention relates to a process for treating steel to give a superior performance in the Weight Loss Test. More particularly this process is especially applicable to steel strip that is electro tin plated and used in the manufacture of carbonated beverage packs.

A Weight Loss Test has been developed to provide an accelerated corrosion test that will assess the performance of steels used in the manufacture of electrolytic tin plate particularly where the contemplated end use is in the manufacture of containers for application in the carbonated beverage packaging field. The essential features of the Weight Loss Test are as follows:

A preweighed sample of steel or detinned and de-alloyed tin plate, 4 square centimetres in area, is immersed in a deaerated test solution of synthetic Cola medium for 96 hours. During the test the reaction vessel and solution is kept deaerated by continuous flushing with carbon dioxide. After the immersion the weight loss is determined, and for convenience the weight loss is converted to micro amps cm. The lower the value obtained the better the tin plate.

Increases in the concentration of sulfur in the steel, as determined by analysis during steelmaking in the Open Hearth furnace, is known to influence the Weight Loss Test values. However it is difficult to control the sulfur content within narrow limits to guarantee specific properties during steelmaking practice. Moreover a general increase in the sulfur content of the steel is undesirable and may give rise to difficulties in the rolling of the steel ingots into steel strip. Furthermore sulfur introduced during steelmaking is prone to segregation and thus will be depicted in the area adjacent to the steel strip surface where its effects are required.

The object of this invention is to provide a process which will produce steel strip that will exhibit consistently low Weight Loss Test values.

Another object of this invention is that the steel strip can be modified without seriously disrupting the major processing steps in the production of electrolytic tin plate from the steelmaking through to the final plating operations.

These objects are achieved by incorporating sulfur into the surface layers of the steel by diffusion during the annealing cycle that is conducted after cold rolling and cleaning operations. The process involves contacting the steel strip with an aqueous solution of sulfide ions, driving off the water from the surface film and annealing the coated steel strip in a neutral or reducing atmosphere.

Following the cold reduction of the steel strip to the required gauge the strip must be cleaned to remove the residual oils remaining on the strip after rolling. This can be achieved by immersion in or spraying of the conventional hot alkaline cleaning solutions based on such materials as sodium hydroxide, sodium carbonate and sodium orthosilicate. The strip should be thoroughly rinsed with water before passing on to the sulfiding step. t

A film containing sulfide is formed on the surface of the steel strip by exposing the cleaned steel strip to an aqueous solution of sulfide ions. The sulfide containing solution may be applied with sprays or by strip immersion in the solution. However it is essential that the space surrounding the application apparatus be adequately ventilated to remove hydrogen sulfide.

The sulfide-containing solution consists of an aqueous solution of sulfide ions in the concentration range 0.15 to 1.5 percent W/W and at a pH of between 7.0 to 9.5. The sulfide ions may be supplied by dissolving in water any of the alkalimetal sulfides to give the required concentration of sulfide ions. The preferred method of preparing the solution is to dissolve sodium sulfide in water to give a solution containing 0.4 percent W/W sulfide ions and then adjust the pH by the addition of sulfuric acid until a piece of clean cold rolled steel, when immersed in the solution, blackens. The H obtained in this way will normally be m the range pH 8 to The solution temperature is maintained in the range 18 to 35 C. for the purposes of the treatment.

The time of contact between the sulfide-containing solution and the steel strip will depend on the particular solution conditions and the degree of effect required, however, in the preferred method treatment times of between 0.5 and 5.0 seconds are used.

Rinsing the strip with water after the sulfide treatment is not essential but is desirable, particularly when solutions of high sulfide concentrations are used.

The strip should then be dried using any of the conventional techniques such as blasting the steel surface with hot air.

To provide a sulfur enriched steel surface the strip is heat treated in a protective atmosphere to effect the diffusion of the sulfur from the surface film into the surface layers of the steel strip. This may be accomplished in the conventional continuous annealing furnaces or in the conventional batch annealing furnaces.

In the batch annealing furnace the coiled steel strip would be heated to between 600 and 800 C. and held for between 0.5 and 12 hours. A neutral or reducing protective atmosphere is maintained around the coiled steel strip throughout the heat treatment to prevent oxidation. The protective atmosphere may be any of those known to the art. An example is ANX gas containing 5% V/V hydrogen, 95% V/V nitrogen and moisture to give a dewpoint below 40 F.

In the continuous annealing furnace the strip is passed into a furnace zone filled with a neutral or reducing protective atmosphere, known to the art, and held at temperatures in the range 600 to 800 C. for between 20 seconds and seconds. The strip is then cooled in the presence of the protective atmosphere.

The steel strip so prepared can then be processed by the conventional techniques to make electrolytic tin plate that will exhibit low Weight Loss Test values.

What we claim is:

l. A process of preparing a steel strip that will exhibit consistently low Weight Loss Test values, which comprises treating a cold rolled, clean steel strip with an aqueous solution containing sulphide ions derived from a sulfide selected from the group consisting of alkali metal sulfides in a concentration in the range of from 0.15 to 1.5 percent W/W and at a pH of between 7 .0 and 9.5 to form a sulfide film on the strip, and then, after drying, heat-treating the steel strip in a neutral or reducing protective atmosphere at a temperature of between 600 and 800 C. sufficiently to cause diffusion of the sulfide film into the surface of the steel strip.

2. A process as claimed in claim 1, wherein the heat treatment is carried out for between 20 and 90 seconds in a continuous annealing furnace.

3. A process as claimed in claim 1, wherein the heat treatment is carried out for between 0.5 and 12 hours in a conventional batch annealing furnace.

4. A process according to claim 1, wherein the alkali metal sulfide is sodium sulfide. 

2. A process as claimed in claim 1, wherein the heat treatment is carried out for between 20 and 90 seconds in a continuous annealing furnace.
 3. A process as claimed in claim 1, wherein the heat treatment is carried out for between 0.5 and 12 hours in a conventional batch annealing furnace.
 4. A process according to claim 1, wherein the alkali metal sulfide is sodium sulfide. 